How to ensure secure access to an authorization token…

There are several methods to consider.

Content Provider with restricted access: Create your own ContentProvider that implements logic for obtaining a token. Restrict access to it using permissions, which will be defined in your application's manifest. Another application must request this permission to access the Content Provider.

<!-- In your application's AndroidManifest.xml -->
<permission android:name="com.your_app.PERMISSION_GET_TOKEN"
            android:label="@string/permission_get_token_label"
            android:description="@string/permission_get_token_description"
            android:protectionLevel="signature" />

<application ...>
    <provider
        android:name=".TokenContentProvider"
        android:authorities="com.your_app.token_provider"
        android:exported="true"
        android:readPermission="com.your_app.PERMISSION_GET_TOKEN"
        ... />
</application>

// Example implementation of ContentProvider
public class TokenContentProvider extends ContentProvider {
    // ... implementation of query() to issue token with permission check ...
}

The other application must declare a request for this permission:

<!-- In the other application's AndroidManifest.xml -->
<uses-permission android:name="com.your_app.PERMISSION_GET_TOKEN" />

The protectionLevel="signature" level guarantees that only an application signed with the same key as yours can access the Content Provider. This is the safest option for data exchange between applications from the same company (same developer).

Service with binding and UID/PackageName verification: Create a Service that provides a method to obtain the token. The other application can bind to this service (bindService). Inside the service, when processing a request, you can get the caller's UID or PackageName and verify if it is trusted.

// Example implementation of Service with AIDL
public class TokenService extends Service {
    private ITokenService.Stub binder = new ITokenService.Stub() {
        @Override
        public String getToken() throws RemoteException {
            // Check the calling application by getCallingUid() or getPackagesForUid()
            String[] packages = getPackageManager().getPackagesForUid(Binder.getCallingUid());
            // Verify packages against a list of allowed ones
            if (isAllowedPackage(packages)) {
                // Return the token (assumed to be stored securely)
                return "your_auth_token";
            } else {
                throw new SecurityException("Unauthorized access");
            }
        }
    };

    @Nullable
    @Override
    public IBinder onBind(Intent intent) {
        return binder;
    }
}

AIDL (Android Interface Definition Language) is used to define the service interface for inter-process communication.

SharedPreferences with MODE_WORLD_READABLE (not recommended): Save the token in SharedPreferences with the MODE_WORLD_READABLE flag. This allows any application to read this file. This method is deprecated and not recommended due to low security.
```
// Unsafe method (deprecated)
SharedPreferences preferences = getSharedPreferences("token_prefs", Context.MODE_WORLD_READABLE);
String token = preferences.getString("auth_token", null);
```
Passing via Intent with restrictions: Pass the token via Intent (e.g., through startActivity with data). However, this is the least secure method, as data in Intent can be intercepted. If you need to pass the token this way, use encrypted data and verify the digital signature or PackageName of the recipient application.
```
// Example (requires additional security measures)
Intent intent = new Intent("com.other_app.ACTION_RECEIVE_TOKEN");
intent.putExtra("token", "your_auth_token"); // Token should be encrypted
startActivity(intent);
```

The choice of method depends on security requirements and the level of trust between your application and the application that needs access to the token. The most secure options are Content Provider and Service with explicit caller verification.

How to ensure secure access to an authorization token for another application?